Zevvle (https://zevvle.com/) actually do a good job of this. You pay per the byte that you use at a given rate and you can see in their easy to use, easy to install app exactly what/how much you have used down to the minute. The important thing is that although you can see the amount in MB/GB it is also translated into £. So you can see how much actual currency you have spent at 6pm this evening watching that YouTube video on the train. Honestly, I think the Telco apps are one of the biggest issues at the moment. I used to be on EE and the app was a bloated piece of garbage - it couldn't show me how much data I had used from month to month, let alone throughout the day.